Shock cords are the lengths of cord that hold the different pieces of the vehicle airframe together during recovery. They are also where the parachutes are attached. Without shock cords, the entire recovery process would not work very well.
There are a number of different factors that should be kept in mind when choosing the right shock cord for different vehicle requirements.
Two of the most popular shock cord materials for rocketry are kevlar and nylon. Each offers unique advantages and disadvantages to consider.
Kevlar is typically stronger than nylon at the same thickness. In fact, it is about five times stronger than even steel. Additionally, kevlar offers great resistance to heat and corrosion, making it useful for applications where black powder is the method of separation.
Compared to nylon, kevlar is much more stiff, meaning it will not stretch as much when force is applied to it. This is relevant when considering how much of the parachute deployment energy will be absorbed by the shock cord versus how much energy will instead be transferred to the attachment points. The increased stiffness also means that kevlar is slightly more difficult to pack into a body tube.
Kevlar is also typically more expensive per unit length than nylon.
Nylon may not be as strong as kevlar for the same thickness, but its primary strength that makes up for that is its elasticity. Nylon shock cord is typically much more "stretchy" than kevlar. This allows it to absorb more of the energy from parachute deployment, which lessens the load on the attachment points.